The present invention relates to an evaporative emission control system for an internal combustion engine, and more particularly to such a system that the emission of evaporative fuel is prevented by maintaining the pressure in a fuel tank at a negative pressure.
For example, Japanese Patent Laid-open No. 11-50919 discloses an evaporative emission control system including an evaporative fuel passage for connecting a fuel tank directly to an intake pipe of an internal combustion engine to maintain the pressure in the fuel tank at a negative pressure (a pressure lower than the atmospheric pressure). This conventional system further includes a tank pressure control valve provided in the evaporative fuel passage. When the pressure in the fuel tank is higher than the pressure in the intake pipe, valve opening control of the tank pressure control valve is enabled to be carried.
In the conventional system mentioned above, however, since the pressure in the intake pipe always varies according to engine operating conditions, there is a case that in the open condition of the tank pressure control valve, the pressure in the intake pipe becomes higher than the pressure in the fuel tank during the period between successive detections of the pressure in the fuel tank, causing an increase in the pressure in the fuel tank.